Metal-insulator-metal capacitor analog memory unit cell

ABSTRACT

A memory device including a plurality of memory unit cells arranged in a crossbar configuration for a neural network is provided. Each of the memory unit cells includes a readout transistor, a charging transistor, a discharging transistor, and a metal-insulator-metal (MIM) capacitor connected to one of source/drain regions of each of the charging transistor and the discharging transistor and a functional gate of the readout transistor for storing analog information.

BACKGROUND

The present application relates to neural network architecture, andparticularly, to a memory unit cell including a metal-insulator-metal(MIM) capacitor for neural networks.

Neural networks have wide applicability in various technical fieldsincluding automatic recognition systems, such as character recognitionsystems, voice recognition systems, etc., activation control systems forrobots and neuro computer systems incorporating artificial intelligence.

Current-based nonvolatile memory (NVM) devices represent states withdifferent resistance values, and are attractive for neural networkacceleration. Current-based memory devices in a crossbar configurationcan implement vector-matrix multiplication in neural networkcomputations. By mapping input vector to input voltages and weightmatrix to resistive crossbar array, vector matrix multiplication can becalculated in a single step by sampling the current flowing in eachcolumn. This approach can be several orders of magnitude more efficientthan CMOS ASIC approaches in terms of both speed and power.

Capacitors have been used as a storage element to store analoginformation in neutral network. Although charge stored on the capacitorscan leak, several schemes have been developed to overcome that problem.For example, a periodic refresh scheme can be employed. Another approachis to guarantee that each neural network learning cycle is shorter thana certain time period, where the capacitor leakage is not significantenough to impact overall neural network training and convergence.Therefore, developing a compact memory unit cell structure with acapacitor as the analog information storage element remains needed forneural network applications.

SUMMARY

The present application provides a memory device including a pluralityof memory unit cells arranged in a crossbar configuration for a neuralnetwork. Each of the unit memory cells includes a readout transistor, acharging transistor, a discharging transistor, and ametal-insulator-metal (MIM) capacitor connected to one of thesource/drain regions of each of the charging transistor and thedischarging transistor and a functional gate of the readout transistorfor storing analog information.

According to an aspect of the present application, a memory cell arrayincluding a plurality of memory unit cells arranged in a crossbarconfiguration is provided. Each of the plurality of memory unit cellsincludes a first field effect transistor (FET) located in a first activeregion of a substrate and having a first functional gate stack thatcontacts a portion of a first semiconductor material portion and firstsource/drain regions located on opposite sides of the first functionalgate stack, a second FET located in a second active region of thesubstrate and having a second functional gate stack that contacts aportion of a second semiconductor material portion and secondsource/drain regions located on opposite sides of the second functionalgate stack, a third FET located in a third active region of thesubstrate and having a third functional gate stack that contacts aportion of a third semiconductor material portion and first source/drainregions located on opposite sides of the third functional gate stack, asource/drain interconnect structure overlying and connecting one of thesecond source/drain regions and one of the third source/drain regions,and a metal-insulator-metal (MIM) capacitor overlying and contacting thesource/drain interconnect structure and a functional gate of the firstfunctional gate stack.

According to another aspect of the present application, a method offorming a memory unit cell is provided. The method includes firstproviding a first FET that includes a first functional gate stackcontacting a portion of a first semiconductor material portion and firstsource/drain regions located on opposite sides of the first functionalgate stack in a first active region of a substrate, a second FET thatincludes a second functional gate stack contacting a portion of a secondsemiconductor material portion and second source/drain regions locatedon opposite sides of the second functional gate stack in a second activeregion of the substrate, and a third FET that includes a thirdfunctional gate stack contacting a portion of a third semiconductormaterial portion and third source/drain regions located on oppositesides of the third functional gate stack in a third active region of thesubstrate. Next, first source/drain contact structures overlying andcontacting the first source/drain regions, a second source/drain contactstructure overlying and contacting one of the second source/drainregions, a third source/drain contact structure overlying and contactingone of the third source/drain regions, and a source/drain interconnectstructure connecting the other of the second source/drain regions andthe other of the third source/drain regions are formed. The first, thesecond and the third source/drain contact structures and thesource/drain interconnect structure are laterally surrounded by aninterlevel dielectric (ILD) layer located over the substrate. Next, ametal-insulator-metal capacitor is formed overlying and contacting thesource/drain interconnect structure and a functional gate of the firstfunctional gate stack.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a circuit diagram of a memory device including a plurality ofmemory unit cells arranged in a crossbar configuration for achievingmatrix-vector multiplication approximation operation.

FIG. 2A is a top-down view of an exemplary semiconductor structureincluding a set of first semiconductor material portions formed in afirst active region of a substrate, a set of second semiconductormaterial portions formed in a second active region of the substrate, anda set of third semiconductor material portions formed in a third activeregion of the substrate according to an embodiment of the presentapplication.

FIG. 2B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 2A along line B-B′.

FIG. 2C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 2A along line C-C′.

FIG. 3A is a top-down view of the exemplary semiconductor structure ofFIGS. 2A-2C after forming a first sacrificial gate stack, a secondsacrificial gate stack and a third sacrificial gate stack and forming agate spacer on sidewalls of each sacrificial gate stack.

FIG. 3B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 3A along line B-B′.

FIG. 3C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 3A along line C-C′.

FIG. 3D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 3A along line D-D′.

FIG. 4A is a top-down view of the exemplary semiconductor structure ofFIGS. 3A-3D after forming first source/drain regions, secondsource/drain regions and third source/drain regions.

FIG. 4B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 4A along line B-B′.

FIG. 4C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 4A along line C-C′.

FIG. 4D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 4A along line D-D′.

FIG. 5A is a top-down view of the exemplary semiconductor structure ofFIGS. 4A-4D after forming an interlevel dielectric (ILD) layer laterallysurrounding the first, the second and the third sacrificial gate stacksand the gate spacers.

FIG. 5B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 4A along line B-B′.

FIG. 5C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 5A along line C-C′.

FIG. 5D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 5A along line D-D′.

FIG. 6A is a top-down view of the exemplary semiconductor structure ofFIGS. 5A-5D after forming a first functional gate stack, a secondfunctional gate stack and a third functional gate stack.

FIG. 6B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 6A along line B-B′.

FIG. 6C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 6A along line C-C′.

FIG. 6D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 6A along line D-D′.

FIG. 7A is a top-down view of the exemplary semiconductor structure ofFIGS. 6A-6D after forming various source/drain contact structures and asource/drain interconnect structure connecting one of the secondsource/drain regions and one of the third source/drain regions.

FIG. 7B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 7A along line B-B′.

FIG. 7C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 7A along line C-C′.

FIG. 7D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 7A along line D-D′.

FIG. 8A is a top-down view of the exemplary semiconductor structure ofFIGS. 7A-7D after forming a MIM stack and an insulator layer on the MIMstack.

FIG. 8B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 8A along line B-B′.

FIG. 8C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 8A along line C-C′.

FIG. 8D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 8A along line D-D′.

FIG. 9A is a top-down view of the exemplary semiconductor structure ofFIGS. 8A-8D after patterning the insulator layer and the MIM stack toform a MIM capacitor and an insulator portion on the MIM capacitor.

FIG. 9B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 9A along line B-B′.

FIG. 9C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 9A along line C-C′.

FIG. 9D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 9A along line D-D′.

FIG. 10A is a top-down view of the exemplary semiconductor structure ofFIGS. 9A-9D after forming a contact level dielectric layer on the ILDlayer to laterally surround the MIM capacitor and the insulator portion.

FIG. 10B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 10A along line B-B′.

FIG. 10C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 10A along line C-C′.

FIG. 10D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 10A along line D-D′.

FIG. 11A is a top-down view of the exemplary semiconductor structure ofFIGS. 10A-10D after forming a first opening exposing a surface of thefunctional gate of the first functional gate stack and sidewalls of theMIM capacitor located at a first end of the MIM capacitor.

FIG. 11B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 11A along line B-B′.

FIG. 11C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 11A along line C-C′.

FIG. 11D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 11A along line D-D′.

FIG. 12A is a top-down view of the exemplary semiconductor structure ofFIGS. 11A-11D after laterally etching physically exposed second metalelectrodes of the MIM capacitor to form first undercut regions.

FIG. 12B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 12A along line B-B′.

FIG. 12C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 12A along line C-C′.

FIG. 12D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 12A along line D-D′.

FIG. 13A is a top-down view of the exemplary semiconductor structure ofFIGS. 12A-12D after forming first dielectric caps to fill the firstundercut regions.

FIG. 13B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 13A along line B-B′.

FIG. 13C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 13A along line C-C′.

FIG. 13D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 13A along line D-D′.

FIG. 14A is a top-down view of the exemplary semiconductor structure ofFIGS. 13A-13D after forming a second opening exposing a surface of theILD layer and sidewalls of the MIM capacitor located at a second end ofthe MIM capacitor wherein the source/drain interconnect structure islocated.

FIG. 14B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 14A along line B-B′.

FIG. 14C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 14A along line C-C′.

FIG. 14D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 14A along line D-D′.

FIG. 15A is a top-down view of the exemplary semiconductor structure ofFIGS. 14A-14D after laterally etching physically exposed first metalelectrodes of the MIM capacitor to form second undercut regions andforming second dielectric caps to fill the second undercut regions.

FIG. 15B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 15A along line B-B′.

FIG. 15C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 15A along line C-C′.

FIG. 15D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 15A along line D-D′.

FIG. 16A is a top-down view of the exemplary semiconductor structure ofFIGS. 15A-15D after forming a first capacitor contact structure withinthe first opening and a second capacitor contact structure within thesecond opening.

FIG. 16B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 16A along line B-B′.

FIG. 16C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 16A along line C-C′.

FIG. 16D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 16A along line D-D′.

FIG. 17A is a top-down view of the exemplary semiconductor structure ofFIGS. 16A-16D after forming various contact structures to FETcomponents.

FIG. 17B is a cross-sectional view of the exemplary semiconductorstructure of FIG. 17A along line B-B′.

FIG. 17C is a cross-sectional view of the exemplary semiconductorstructure of FIG. 17A along line C-C′.

FIG. 17D is a cross-sectional view of the exemplary semiconductorstructure of FIG. 17A along line D-D′.

DETAILED DESCRIPTION

The present application will now be described in greater detail byreferring to the following discussion and drawings that accompany thepresent application. It is noted that the drawings of the presentapplication are provided for illustrative purposes only and, as such,the drawings are not drawn to scale. It is also noted that like andcorresponding elements are referred to by like reference numerals.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide an understanding ofthe various embodiments of the present application. However, it will beappreciated by one of ordinary skill in the art that the variousembodiments of the present application may be practiced without thesespecific details. In other instances, well-known structures orprocessing steps have not been described in detail in order to avoidobscuring the present application.

FIG. 1 is a circuit diagram of a memory device including a plurality ofmemory unit cells 100 arranged in a crossbar configuration for achievingmatrix-vector multiplication approximation operation. The crossbarincludes a plurality of voltage lines, e.g., V1, V2, V3, V4 rows, and aplurality of current lines, I1, I2, I3, I4 columns. A set of inputvoltages V_(j) are applied on voltage lines, and the current I_(i) iscollected through each current line. The current flowing through eachcurrent line is equal to the sum of the product of an input voltageV_(j) and a crossbar weight value stored as the conductance g_(ij),

$I_{i} = {\sum\limits_{j}{g_{ij} \times {V_{j}.}}}$

Each memory cell 100 in the memory device may include a chargingtransistor 102, a discharging transistor 104, a readout transistor 106and a capacitor 108. In the present embodiment, the charging transistor102 may be implemented as a p-type field effect transistor (pFET), thedischarging transistor 104 may be implemented as an n-type FET (nFET),and the readout transistor 106 may be implemented as an nFET or a pFET.The charging transistor 102 has a source coupled to a supply voltageV_(dd), a gate coupled to a first input line, and a drain coupled tonode 114 that is coupled to a drain of the discharging transistor 104. Agate of the discharging transistor 104 is coupled to a second input lineand a source of the discharging transistor 104 is coupled to ground. Thereadout transistor 106 has a source coupled to a voltage line (e.g., atpoint A), a drain coupled to a current line (e.g.) at point B and a gatecoupled to node 116 that is coupled to node 114 and a first terminal ofthe capacitor 108. A second terminal of the capacitor 108 is coupled toground.

The capacitor 108 thus is electrically connected to node 114 to receivea charging current from the charging transistor 102. As a result, avoltage at node 114 gradually increases due to the charging activity ofthe charging current. The capacitor 108 is discharged through thedischarging transistor 104, causing the voltage at node 114 to dropdown. The capacitor voltage is thus adjusted by charging anddischarging. As the voltage of the capacitor 108 changes, theconductance of the readout transistor 106 changes continuously.

FIGS. 2-17 illustrate an exemplary method for fabricating a memory cell100 described in FIG. 1.

Referring to FIGS. 2A-2C, there are illustrated various views of anexemplary semiconductor structure that can be employed according to anembodiment of the present application. The exemplary semiconductorstructure includes a set of first semiconductor material portions 16Aformed in a first active region (AT1) of a substrate 8, a set of secondsemiconductor material portions 16B formed in a second active region(AT2) of the substrate 8, and a set of third semiconductor materialportions 16C formed in a third active region (AT3) of the substrate 8.

The active regions, i.e., areas where active FETs are to be formed, arearranged side by side such that the second active region AT2 is locatedadjacent to the first active region AT1 along a first direction X1, andadjacent to the third active region AT3 along a second direction Y1 thatis perpendicular to the first direction X1. In one embodiment, the firstand second active regions AT1, AT2 are nFET regions and the third activeregion AT3 is a pFET region. In some embodiments, the first activeregion AT1 can also be a pFET region.

In one embodiment, each semiconductor material portion 16A, 16B, 16C maybe a semiconductor fin. The term “semiconductor fin” denotes acontiguous semiconductor structure that extends upward from a surface ofa substrate (e.g., substrate 8) and has a pair of vertical sidewallsthat are parallel to each other. As used herein, a surface is “vertical”if there exists a vertical plane from which the surface does not deviateby more than three times the root mean square roughness of the surface.The semiconductor fins are aligned in their lengthwise directions (i.e.,along the first direction X1 and so the first direction X1 may bereferred to hereinafter as the lengthwise direction of the fins) andspaced from each other in their widthwise directions (i.e., in thesecond direction Y1 so the second direction Y1 may also be referred tohereinafter as the widthwise direction of the fins). The height andwidth of the semiconductor fins can vary. For example, eachsemiconductor fin may have a height ranging from 10 nm to 100 nm, and awidth ranging from 4 nm to 30 nm. In another embodiment, eachsemiconductor material portion 16A, 16B, 16C may be a semiconductornanowire for forming nanowire FETs. In a further embodiment, eachsemiconductor material portion 16A, 16B, 16C may be a planar activesemiconductor region for forming planar FETs.

In one embodiment, the semiconductor material portions 16A, 16B, 16C maybe formed by providing a semiconductor-on-insulator (SOI) substrateincluding, from bottom to top, a handle substrate 10, a buried insulatorlayer 12, and a top semiconductor layer (not shown), and by patterningthe top semiconductor layer. In this case, the buried insulator layer 12and the handle substrate 10 constitute the substrate 8.

The handle substrate 10 may include a semiconductor material, such as,for example, Si, SiGe, SiGeC, SiC, an III-V compound semiconductor, anII-VI compound semiconductor, or any combinations thereof. Multilayersof semiconductor materials can also be used as the semiconductormaterial of the handle substrate 10. In one embodiment, the handlesubstrate 10 is composed of single crystalline silicon. The thickness ofthe handle substrate 10 can be from 50 μm to 2 mm, although lesser andgreater thicknesses can also be employed.

The buried insulator layer 12 that is located on the handle substrate 10may include a dielectric material such as silicon dioxide, siliconnitride, silicon oxynitride, or a combination thereof. The buriedinsulator layer 12 may be formed using a deposition process including,but not limited to, chemical vapor deposition (CVD), plasma enhancedchemical vapor deposition CVD (PECVD), or physical vapor deposition(PVD). Alternatively, the buried insulator layer 12 may be formed bythermal nitridation and/or thermal oxidation of a surface portion of thehandle substrate 10. The buried insulator layer 12 may also be formed byimplanting oxygen atoms into a bulk semiconductor substrate andthereafter annealing the structure. The thickness of the buriedinsulator layer 12 can be from 100 nm to 300 nm, although lesser andgreater thicknesses can also be employed.

The top semiconductor layer may include any semiconductor material asmentioned above for the handle substrate 10. Exemplary semiconductormaterials that can be employed as the top semiconductor layer include,but are not limited to, Si, Ge, SiGe, SiC, and SiGeC, and III/V compoundsemiconductors such as, for example, InAs, GaAs, and InP. Thesemiconductor materials of the top semiconductor layer and the handlesubstrate 10 may be the same or different. In one embodiment, the topsemiconductor layer includes single crystalline silicon. The topsemiconductor layer may be formed by CVD or PECVD. The thickness of thetop semiconductor layer can be from 20 nm to 100 nm, although lesser orgreater thicknesses can also be employed. Alternatively, the topsemiconductor layer may be formed using a Smart Cut process where twosemiconductor wafers are bonded together with an insulator in between.

The top semiconductor layer may be patterned using lithography andetching to provide the semiconductor material portions 16A, 16B, 16C.For example, a photoresist layer (not shown) can be applied over the topsemiconductor layer and lithographically patterned to define a set ofareas covered by a patterned photoresist layer. The top semiconductorlayer is then patterned by an anisotropic etch using the patternedphotoresist layer as an etch mask. In one embodiment, a dry etch suchas, for example, reactive ion etch (RIE) can be used. In anotherembodiment, a wet etch using a chemical etchant can be used. In still afurther embodiment, a combination of dry etch and wet etch can be used.After formation of the semiconductor material portions 16A, 16B, 16C,the patterned photoresist layer can be removed, for example, by ashing.Other methods known in the art, such as sidewall image transfer (SIT) ordirectional self-assembly (DSA), can also be used to pattern the topsemiconductor layer of the SOI substrate to provide the semiconductormaterial portions 16A, 16B, 16C.

In another embodiment, the semiconductor material portions 16A, 16B, 16Cmay be formed by providing a bulk semiconductor substrate including asemiconductor material throughout, and by patterning an upper portion ofthe bulk semiconductor substrate. In this case, the remaining portion ofthe bulk semiconductor substrate underlying the semiconductor materialportions 16A, 16B, 16C constitutes the substrate 8. The semiconductormaterial portions 16A, 16B, 16C are adjoined to the substrate 8 and arecomprised of the same semiconductor material as mentioned above for thetop semiconductor layer of the SOI substrate.

Referring to FIGS. 3A-3D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 2A-2C after formingsacrificial gate stacks. The sacrificial gate stacks include a firstsacrificial gate stack 20A straddling a channel region of each firstsemiconductor material portion 16A, a second sacrificial gate stack 20Bstraddling a channel region of each second semiconductor materialportion 16B, and a third sacrificial gate stack 20C straddling a channelregion of each third semiconductor material portion 16C. By “straddling”it is meant that the sacrificial gate stack formed in each active regionis in direct contact with a top surface and two vertical sidewalls ofthe channel region of each semiconductor material portion. The term“sacrificial gate stack” is used throughout the present application todenote a material stack that serves as a placeholder material for afunctional gate stack to be subsequently formed. The term “functionalgate stack” as used herein refers to a permanent gate stack used tocontrol output current (i.e., flow of carriers in the channel) of asemiconducting device through electrical fields or magnetic fields.

Each sacrificial gate stack 20A, 20B, 20C may include, from bottom totop, a sacrificial gate 22 and a sacrificial gate cap 24. Thesacrificial gate 22 may include a sacrificial gate dielectric (notshown) and a sacrificial gate electrode (not shown).

The sacrificial gate stacks 20A, 20B, 20C may be formed by firstproviding a sacrificial material stack (not shown) that includes, frombottom to top, a sacrificial gate dielectric layer, a sacrificial gateelectrode layer and a sacrificial gate cap layer over each semiconductormaterial portion 16A, 16B, 16C and the substrate 8, and by subsequentlypatterning the sacrificial material stack.

The sacrificial gate dielectric layer may include silicon oxide, siliconnitride, or silicon oxynitride. The sacrificial gate dielectric layermay be formed utilizing a conventional deposition process such as, forexample, CVD or PVD. The sacrificial gate dielectric layer may also beformed by conversion of a surface portion of the semiconductor materialportions 16A, 16B, 16C utilizing thermal oxidation or nitridation. Thethickness of the sacrificial gate dielectric layer can be from 1 nm to10 nm, although lesser and greater thicknesses can also be employed. Insome embodiments of the present application, the sacrificial gatedielectric layer can be omitted.

The sacrificial gate electrode layer may include a semiconductormaterial such as polysilicon or a silicon-containing semiconductor alloysuch as SiGe. The sacrificial gate electrode layer may be formedutilizing a conventional deposition process such as, for example, CVD orPECVD. The thickness of the sacrificial gate electrode layer can be from20 nm to 300 nm, although lesser and greater thicknesses can also beemployed.

The sacrificial gate cap layer may include a dielectric material such asan oxide, a nitride or an oxynitride. In one embodiment, the sacrificialgate cap layer is composed of silicon nitride. The sacrificial gate caplayer may be formed utilizing a conventional deposition process such as,for example, CVD or PECVD. The sacrificial gate cap layer that is formedmay have a thickness from 10 nm to 200 nm, although lesser and greaterthicknesses can also be employed.

The sacrificial material stack may be patterned by lithography andetching. For example, a photoresist layer (not shown) may be appliedover the topmost surface of the sacrificial material stack andlithographically patterned by lithographic exposure and development. Thepattern in the photoresist layer is sequentially transferred into thesacrificial material stack by at least one anisotropic etch. Theanisotropic etch can be a dry etch such as, for example, RIE, a wet etchor a combination thereof. Each remaining portion of the sacrificial gatedielectric layer within a sacrificial gate stack 20A, 20B, 20Bconstitutes a sacrificial gate dielectric, each remaining portion of thesacrificial gate electrode layer within a sacrificial gate stack 20A,20B, 20B constitutes a sacrificial gate electrode, and each remainingportion of the sacrificial gate cap layer within a sacrificial gatestack 20A, 20B, 20B constitutes a sacrificial gate cap. The remainingphotoresist layer can be subsequently removed by, for example, ashing.

After forming the sacrificial gate stacks 20A, 20B, 20C, a gate spacer26 is formed on sidewalls of each sacrificial gate stack 20A, 20B, 20C.Collectively, the first sacrificial gate stack 20A and a correspondinggate spacer 26 present on the sidewalls of the first sacrificial gatestack 20A can be referred to as a first sacrificial gate structure, thesecond sacrificial gate stack 20B and a corresponding gate spacer 26present on the sidewalls of the second sacrificial gate stack 20B can bereferred to as a second sacrificial gate structure, and the thirdsacrificial gate stack 20C and a corresponding gate spacer 26 present onthe sidewalls of the third sacrificial gate stack 20C can be referred toas a third sacrificial gate structure.

Each gate spacer 26 may include a dielectric material such as, forexample, an oxide, a nitride, an oxynitride, or any combination thereof.In one embodiment, each gate spacer 26 is composed of silicon nitride.The gate spacers 26 may be formed by first providing a conformal gatespacer material layer (not shown) on physically exposed surfaces of eachsacrificial gate stack 20A, 20B, 20C, each semiconductor materialportion 16A, 16B, 16C and the substrate 8 and then etching the gatespacer material layer to remove horizontal portions of the gate spacermaterial layer. The gate spacer material layer may be provided by adeposition process including, for example, CVD, PECVD or atomic layerdeposition (ALD). The etching of the gate spacer material layer may beperformed by a dry etch process such as, for example, RIE. A remainingvertical portion of the gate spacer material layer that is present onthe sidewalls of each sacrificial gate stack 20A, 20B, 20C constitutes agate spacer 26. The width of each gate spacer 26, as measured at thebase, can be from 5 nm to 100 nm, although lesser and greater widths canalso be employed.

Referring to FIGS. 4A-4D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 3A-3D after formingsource/drain regions. The source/drain regions include a first sourceregion and a first drain region (collectively referred to as firstsource/drain regions 28A) formed on opposite sides of the firstsacrificial gate structure (20A, 26), a second source region and asecond drain region (collectively referred to as second source/drainregions 28B) formed on opposite sides of the second sacrificial gatestructure (20B, 26) and a third source region and a third drain region(collectively referred to as third source/drain regions 28C) formed onopposite sides of the third sacrificial gate structure (20C, 26). Thefirst source/drain regions 28A merge the first semiconductor materialportions 16A, the second source/drain regions 28B merge the secondsemiconductor material portions 16B, and the third source/drain regions28C merge the third semiconductor material portions 16C.

In one embodiment and when the first and second source/drain regions28A, 28B are implemented for nFETs and the third source/drain regions28C are implemented for a pFET, the first and second source/drainregions 28A, 28B may be formed by a first selective epitaxial growthprocess. The term “epitaxial growth and/or deposition” means the growthof a semiconductor material on a deposition surface of a semiconductormaterial, in which the semiconductor material being grown has the same(or nearly the same) crystalline characteristics as the semiconductormaterial of the deposition surface. For example, a first mask layer (notshown) is applied over the structure of FIGS. 3A-3D and lithographicallypatterned to form a first mask (not shown) that covers the third activeregion AT3, while exposing the first and second active regions AT1, AT2.The first mask layer can be a photoresist layer or a photoresist layerin conjunction with hardmask layer(s). The first and second source/drainregions 28A, 28B are then formed by epitaxially depositing a firstsemiconductor material on physically exposed surfaces of the firstsemiconductor material portions 16A that are not covered by the firstsacrificial gate structure (20A, 26) and the second semiconductormaterial portions 16B that are not covered by the second sacrificialgate structure (20B, 26), but not on dielectric surfaces such as thesurfaces of the sacrificial gate caps 24 and the gate spacers 26, theburied insulator layer 12 and the first mask. The first semiconductormaterial that provides the first and second source/drain regions 28A,28B may include Si or carbon doped silicon (Si:C). The first and secondsource/drain regions 28A, 28B also contain n-type dopants for formationof nFETs. Examples of n-type dopants include, but are not limited to, P,As or Sb. Dopants may be introduced into the first and secondsource/drain regions 28A, 28B by in-situ doping during the firstepitaxial growth process or by ex-situ doping after the first epitaxialgrowth process utilizing ion implantation. After formation of the firstand second source/drain regions 28A, 28B, the first mask can be removed,for example, by oxygen-based plasma etching.

Similarly, the third source/drain regions 28C may be formed by a secondselective epitaxial growth process. For example, a second mask (notshown) is first formed to cover the first and second active regions AT1,AT2, while exposing the third active region AT3. A second semiconductormaterial is then epitaxially deposited on physically exposed surfaces ofthe third semiconductor material portions 16C that are not covered bythe third sacrificial gate structure (20C, 26). The second semiconductormaterial that provides the third source/drain regions 28C may includeSiGe. The third source/drain regions 28C contain p-type dopants forformation of a pFET. Examples of p-type dopants include, but are notlimited to, B, Al, Ga or In. Dopants may be introduced into the thirdsource/drain regions 28C by in-situ doping during the second epitaxialgrowth process or by ex-situ doping after the second epitaxial growthprocess utilizing ion implantation. After formation of the thirdsource/drain regions 28C, the second mask can be removed, for example,by oxygen-based plasma etching.

In another embodiment and when the first and third source/drain regions28A, 28C are implemented for pFETs and the second source/drain regions28C are implemented for an nFET, the first and third source/drainregions 28A, 28 c may be formed by the second selective epitaxial growthprocess described above and the second source/drain regions 28C may beformed by the first selective epitaxial growth process described above.

Referring to FIGS. 5A-5D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 4A-4D after forming aninterlevel dielectric (ILD) layer 30 over the source/drain regions 28A,28B, 28C and the buried insulator layer 12. The ILD layer 30 laterallysurrounds the sacrificial gate stacks 20A, 20B, 20C and the gate spacers26. The ILD layer 30 may include a dielectric material that can beeasily planarized. For example, the ILD layer 30 can be a doped silicateglass, an undoped silicate glass (silicon oxide), an organosilicateglass (OSG), or a porous dielectric material. The ILD layer 30 may beformed by CVD, PVD or spin coating. The ILD layer 30 can be initiallyformed such that an entirety of the top surface of the ILD layer 30 isformed above the topmost surfaces of the sacrificial gate stacks 20A,20B, 20C (i.e., top surfaces of the sacrificial gate caps 24). The ILDlayer 30 can be subsequently planarized, for example, by chemicalmechanical planarization (CMP) and/or a recess etch using thesacrificial gate caps 24 as a polishing and/or an etch stop. After theplanarization, the ILD layer 30 has a top surface coplanar with the topsurfaces of the sacrificial gate caps 24.

Referring to FIGS. 6A-6D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 5A-5D after replacing thefirst sacrificial gate stack 20A, the second sacrificial gate stack 20Band the third sacrificial gate stack 20C with a first functional gatestack 40A, a second functional gate stack 40B and a third functionalgate stack 40C, respectively. As shown, the first functional gate stack40A is formed in the first active region AT1 straddling the channelregion of each first semiconductor material portion 16A, the secondfunctional gate stack 40B is formed in the second active region AT2straddling the channel region of each second semiconductor materialportion 16B, and the third functional gate stack 40C is formed in thethird active region AT3 straddling the channel region of each thirdsemiconductor material portion 16C. Each functional gate stack 40A, 40B,40C is laterally surrounded by a gate spacer 26. Collectively, one ofthe functional gate stacks 40A, 40B, 40C and a corresponding gate spacer26 can be referred to as a functional gate structure.

Each function gate stack 40A, 40B, 40C, may include, from bottom to top,a functional gate 42 and a gate cap 44. As is shown, the top surface ofeach gate cap 44 is coplanar with the top surface of the ILD layer 30.The functional gate 42 may include a U-shaped gate dielectric (notshown), and a gate electrode (not shown) surrounded by the gatedielectric. By “U-shaped” it is meant a structure that includes ahorizontal portion and two vertical extending portions that extendupwards from each end of the horizontal portion.

The gate dielectric may include any gate dielectric material. The gatedielectric material that provides the gate dielectric can be an oxide,nitride, and/or oxynitride. In one example, the gate dielectric materialthat provides the gate dielectric 42 can be a high-k material having adielectric constant greater than silicon dioxide. Exemplary high-kdielectrics include, but are not limited to, HfO₂, ZrO₂, La₂O₃, Al₂O₃,TlO₂, SrTiO₃, LaAlO₃, Y₂O₃, HfO_(x)N_(y), ZrO_(x)N_(y), La₂O_(x)N_(y),Al₂O_(x)N_(y), TiO_(x)N_(y), SrTiO_(x)N_(y), LaAlO_(x)N_(y),Y₂O_(x)N_(y), SiON, SiN_(x), a silicate thereof, and an alloy thereof.Each value of x is independently from 0.5 to 3 and each value of y isindependently from 0 to 2. In some embodiments, a multilayered gatedielectric structure comprising different gate dielectric materials,e.g., silicon dioxide, and a high-k gate dielectric may be formed andused as the gate dielectric. The gate dielectric may be formed by anydeposition process including, for example, CVD, PECVD, PVD or ALD. Thethickness of the gate dielectric can be from 1 nm to 10 nm, althoughlesser and greater thicknesses can also be employed.

The gate electrode may include a conductive metal such as, for example,Al, Au, Ag, Cu, Co or W. The gate electrode may be formed utilizing adeposition process including, for example, CVD, PECVD, PVD or ALD.

Optionally, in some embodiments of the present application, a U-shapedwork function metal (not shown) may be formed between the gatedielectric and the gate electrode to tune the threshold voltage of theFETs to be formed. The work function metal may be formed, for example,by CVD, PVD, or ALD. The thickness of the work function metal can befrom 1 nm to 10 nm, although lesser or greater thicknesses can also beemployed.

The replacement of each sacrificial gate stack 20A, 20B, 20C includesremoving components of each sacrificial gate stack 20A, 20B, 20Cutilizing an etching process and thereafter a functional gate stack 40A,40B, 40C is formed in a gate cavity that comprises a volume of eachremoved sacrificial gate stack 20A, 20B, 20C. The formation of eachfunctional gate stack 40A, 40B, 40C includes depositing a gatedielectric material and a gate electrode material that provide eachfunctional gate stack 40A, 40B, 40C and performing a planarizationprocess such as, for example, CMP, to remove the deposited gatedielectric material and gate electrode material from above the topsurface of the ILD layer 30. After planarization, the deposited gatedielectric material and gate electrode material are recessed to providea gate dielectric and a gate electrode (i.e., a functional gate 42)within each gate cavity. A dielectric material is then deposited on topof the functional gate 42 to provide a gate cap 44 within each gatecavity.

Thus, a first FET is formed in the first active region AT1 of thesubstrate 8, a second FET is formed in the second active region AT2 ofthe substrate 8, and a third FET is formed in the third active regionAT3 of the substrate 8. In one embodiment of the present application,the first FET can be an nFET or a pFET, the second FET can be an nFET,and the third FET can be a pFET. The first FET is implemented as thereadout transistor, the second FET is implemented as the dischargingtransistor and the third FET is implemented as the charging transistor.The first FET includes a first functional gate structure containing afirst functional gate stack 40A straddling a channel region of eachfirst semiconductor material portion 16A and a corresponding gate spacer26 present on sidewalls of the first function gate stack 40A, and firstsource/drain regions 28A present on opposite sides of the firstfunctional gate structure (40A, 26) merging the first semiconductormaterial portion 16A. The second FET includes a second functional gatestructure containing a second functional gate stack 40B straddling achannel region of each second semiconductor material portion 16B and acorresponding gate spacer 26 present on sidewalls of the second functiongate stack 40B, and second source/drain regions 28B present on oppositesides of the second functional gate structure (40B, 26) and merging thesecond semiconductor material portions 16B. The third FET includes athird functional gate structure containing a third functional gate stack40C straddling a channel region of each third semiconductor materialportion 16C and a corresponding gate spacer 26 present on sidewalls ofthe third function gate stack 40C, and third source/drain regions 28Cpresent on opposite sides of the third functional gate structure (40B,26) and merging the third semiconductor material portions 16C.

Referring to FIGS. 7A-7D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 6A-6D after forming varioussource/drain contact structures and a source/drain interconnectstructure 48 over source/drain regions 28A, 28B, 28C. The source/draincontact structures include first source/drain contact structures 46Acontacting the first source/drain regions 28A, a second source/draincontact structure 46B contacting one of the second source/drain regions28B located on a first side of the second functional gate stack 40B, athird source/drain contact structure 46C contacting one of the thirdsource/drain regions 28C located on a first side of the third functionalgate stack 40C. The source/drain interconnect structure 48 connects theother of the second source/drain regions 28B located on a second side ofthe second functional gate stack 40B opposite the first side and theother of the third source/drain regions 28C located on a second side ofthe third functional gate stack 40C, 26 opposite the first side. In oneembodiment, the first side of each of the second functional gate stack40B and the third functional gate stack 40C is distal from the firstfunctional gate stack 40A, and the second side of each of the secondfunctional gate stack 40B and the third functional gate stack 40C isproximal to the first functional gate stack 40A.

The source/drain contact structures 46A, 46B, 46C and the source/draininterconnect structure 48 can be formed by first forming source/draincontact openings (not shown) and a source/drain interconnect opening(not shown) extending through the ILD layer 30, exposing a portion of acorresponding source/drain region 28A, 28B, 28C. The source/draincontact and interconnect openings can be formed by applying a mask layer(not shown) over the ILD layer 30, the functional gate stacks 40A, 40B,40C and the gate spacers 26, and then lithographically patterning themask layer to form openings therein. Each opening overlies a portion ofa corresponding source/drain region 28A, 28B, 28C. The mask layer can bea photoresist layer or a photoresist layer in conjunction with hardmasklayer(s). The pattern in the mask layer is transferred through the ILDlayer 30 by an anisotropic etch to form the source/drain contactopenings and the source/drain interconnect opening. The anisotropic etchcan be a dry etch such as RIE or a wet etch that etches the ILD layer 30selective to the source/drain regions 28A, 28B, 28C. After forming thesource/drain contact openings and the source/drain interconnect opening,the remaining mask layer can be removed by oxygen-based plasma etching.

Subsequently, a conductive material is deposited within the source/draincontact openings and the source/drain interconnect opening. Theconductive material may include a metal such as, for example, W, Al, Cuor their alloys. The conductive material may be deposited by anysuitable deposition method such as, for example, CVD, PVD or plating.Excess portions of the deposited conductive material can be removed fromabove the top surface of the ILD layer 30 by a planarization processsuch as, for example, CMP to provide the final source/drain contactstructures 46A, 46B, 46C and the source/drain interconnect structure 48.Optionally, contact liners (not shown) may be formed on the sidewallsand bottom surfaces of the source/drain contact openings and thesource/drain interconnect opening before filling the openings with theconductive material. In one embodiment, the contact liners may includeTiN.

Referring to FIGS. 8A-8D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 7A-7D after forming a MIMstack on the source/drain contact structures 46A, 46B, 46C, thesource/drain interconnect structure 48, the functional gate stacks 40A,40B, 40C, the gate spacers 26 and the ILD layer 30. The MIM stack mayinclude alternating first metal layers 52 and second metal layers 56that are separated from each other by capacitor dielectric layers 54.The MIM stack (52, 54, 56) may include any number of first and secondmetal layers 52, 56. In one embodiment and as shown, the MIM stack (52,54, 56) includes two first metal layers 52, two second metal layers 56and three capacitor dielectric layers 54 isolated each first metal layer52 from the next adjacent second metal layer 56. All capacitordielectric layers 54 of the MIM stack can be the same dielectricmaterial, or some or each capacitor dielectric layer 54 can comprisedifferent dielectric materials. Similarly, all capacitor dielectriclayers 54 can be formed by the same deposition process, or some or eachcapacitor dielectric layer 54 can be formed by different depositionprocesses. All first metal layers 52 can be, but are not necessarily,the same material, so long as all of the first metal layers 52 can beselectively etched relative to all the second metal layers by a singleetch. Similarly, all second metal layers 56 can be, but are notnecessarily, the same material, so long as all of the second metallayers 56 can be selectively etched relative to all the first metallayers 52 by a single etch.

Each first metal layer 52 may include a conductive metal such as, forexample, TiN, TaN, Ti, Ta, Cu, Ru, or the like. Each first metal layer52 may be formed utilizing a deposition process including, for example,CVD, PECVD, PVD or ALD. The thickness of each first metal layer 30 canbe from 1 nm to 25 nm, although lesser and greater thicknesses can alsobe employed.

Each capacitor dielectric layer 54 may include a high-k dielectricmaterial (i.e., a dielectric material having a dielectric constant thatis greater than the dielectric constant of silicon dioxide). Exemplarycapacitor dielectrics include, but are not limited to, HfO₂, ZrO₂,La₂O₃, Al₂O₃, TiO₂, SrTiO₃, LaAlO₃, Y₂O₃, HfO_(x)N_(y), ZrO_(x)N_(y),La₂O_(x)N_(y), Al₂O_(x)N_(y), TiO_(x)N_(y), SrTiO_(x)N_(y),LaAlO_(x)N_(y), Y₂O_(x)N_(y), SiON, SiN_(x), a silicate thereof and analloy thereof. Each value of x is independently from 0.5 to 3 and eachvalue of y is independently from 0 to 2. Each capacitor dielectric layer54 may be formed by any conformal deposition process including, forexample, CVD, PECVD or ALD. The thickness of each capacitor dielectriclayer 66 can be from 1 nm to 10 nm, although lesser and greaterthicknesses can also be employed.

Each second metal layer 56 may include a conductive metal different fromthe conductive metal that provides each first metal layer 52. Forexample, each second metal layer 56 may include TiN, TaN, Ti, Ta, Cu,Ru, or the like. Each second metal layer 56 may be formed utilizing adeposition process including, for example, CVD, PECVD, PVD or ALD. Thethickness of each second metal layer 56 can be from 1 nm to 25 nm,although lesser and greater thicknesses can also be employed.

After formation of the MIM stack (52, 54, 56), an insulator layer 58 isdeposited on the topmost surface of the MIM stack (52, 54, 56). Theinsulator layer 58 may include a dielectric nitride such as, forexample, silicon nitride, SiBCN, SiOCN or SiCN. The insulator layer 58may be formed by CVD, PECVD or ALD. The thickness of the insulator layer58 can be from 30 nm to 100 nm, although lesser and greater thicknessescan also be employed.

Referring to FIGS. 9A-9D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 8A-8D after patterning theinsulator layer 58 and the MIM stack (52, 54, 56) to form a MIMcapacitor overlying and contacting the source/drain interconnectstructure 48 and the functional gate 42 of the first functional gatestack 40A and an insulator portion 58) on top of the MIM capacitor. TheMIM capacitor includes alternating first metal electrodes 52P and thesecond metal electrodes 56P, and capacitor dielectrics 54P insulatingthe first metal electrodes 52P and the second metal electrodes 56P fromeach other. In one embodiment and as shown, the MIM capacitor (52P, 54P,56P) is L-shaped having a first leg that extends along the firstdirection X1 and contacts the functional gate 42 of the first functionalgate stack 40A and a second leg that extends along the second directionY1 and contacts the source/drain interconnect structure 48. It should benoted that although an L-shape MIM capacitor is illustrated in FIG. 9Aand described above, any capacitor shape that tends to maximize theeffective area of the MIM capacitor can be employed.

The patterning of the insulator layer 58 and the MIM stack can beperformed by first applying a photoresist layer (not shown) on top ofthe insulator layer 58 and lithographically patterning the photoresistlayer such that a remaining portion of the photoresist layer, i.e., apatterned photoresist layer (not shown), only covers the source/draininterconnect structure 48 and a portion of the first functional gatestack 40A located over the buried insulator layer 12. Subsequently,portions of the insulator layer 58 and the MIM stack (52, 54, 56) thatare not covered by the patterned photoresist layer are removed by atleast one anisotropic etch, which can be a dry etch such as RIE or a wetetch. After etching, a remaining portion of the insulator layerconstitutes the insulator portion, a remaining portion of each firstmetal layer 52 constitutes a first metal electrode 52P, a remainingportion of each capacitor dielectric layer 54 constitutes a capacitordielectric 54P, and a remaining portion of each second metal layer 56constitutes a second metal electrode 56P.

Referring to FIGS. 10A-10D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 9A-9D after forming a contactlevel dielectric layer 60 on the ILD layer 30, the functional gatestacks 40A, 40B, 40C, the gate spacers 26 and the source/drain contactstructures 46A, 46B, 46C. The contact level dielectric layer 60 mayinclude a dielectric material the same as, or different from, thedielectric material that provides the ILD layer 30. For example, thecontact level dielectric layer 60 can be a doped silicate glass, anundoped silicate glass (silicon oxide), an organosilicate glass (OSG),or a porous dielectric material. The contact level dielectric layer 60may be formed by CVD, PVD or spin coating. After deposition, the contactlevel dielectric layer 60 may be subsequently planarized, for example,by CMP using the insulator portion 58P as a polishing stop. The contactlevel dielectric layer 60 laterally surrounds the MIM capacitor (52P,54P, 56P) and the insulator portion 58P and has a top surface coplanarwith the top surface of the insulator portion 58P.

Referring to FIGS. 11A-11D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 10A-10D after forming a firstopening 62 extending through the insulator portion 58P, the MIMcapacitor (52P, 54P, 56P) and the gate cap 44 of the first functionalgate stack 40A to expose a surface of the functional gate 42 of thefirst functional gate stack 40A. The first opening 62 also exposessidewalls of the MIM capacitor (52P, 54P, 56P) located at a first end ofthe MIM capacitor (52P, 54P, 56P) (i.e., an end of the first leg of theMIM capacitor)).

The first opening 62 can be formed by first applying a mask layer (notshown) on the contact level dielectric layer 60 and the insulatorportion 58P and lithographically patterning the mask layer to form apatterned mask layer having an opening therein. The mask layer can be aphotoresist layer or a photoresist layer in conjunction with a hardmasklayer(s). A portion of the insulator portion 58P that is exposed by theopening within the patterned mask layer and the underlying MIM capacitor(52P, 54P, 56P) and the gate cap 44 of the first functional gate stack40A are then removed by at least one anisotropic etch. The etch can be adry etch such as RIE or a wet etch. After etching, the patterned masklayer can be removed, for example, by oxygen-based plasma etching.

Referring to FIGS. 12A-12D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 11A-11D after slightlyrecessing physically exposed sidewalls of the second metal electrodes56P to form first undercut regions 64 between adjacent dielectricsincluding adjacent capacitor dielectrics 54P as well as adjacentinsulator portion 58 and capacitor dielectric 54P. Sidewalls of thesecond metal electrodes 56P are thus offset from sidewalls of thecapacitor dielectrics 54P. An isotropic etch can be performed tolaterally etch the second metal electrodes 56P selective to the firstmetal electrodes 52P, the capacitor dielectrics 54P and the insulatorportion 58P. The isotropic etch can be a dry etch such as RIE or a wetetch.

Referring to FIGS. 13A-13D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 12A-12D after forming a firstdielectric cap 66 to fill each first undercut region 64. Each firstdielectric cap 66 has an outer sidewall vertically aligned with thesidewalls of the capacitor dielectrics 54P.

The first dielectric caps 66 may be formed by first forming a conformaldielectric layer (not shown) over physically exposed surfaces of the ILDlayers 30, the contact level dielectric layer 60, the first functionalgate stack 40A, the gate spacer 26, the insulator portion 58P and theMIM capacitor (52P, 54P, 56P). The conformal dielectric layer mayinclude a dielectric material having a gap filling property, therebyfilling the first undercut regions 64. In one embodiment, the conformaldielectric layer may include a dielectric oxide such as, for example, ahigh density plasma oxide. The conformal dielectric layer may bedeposited by a conformal deposition process such as, for example, CVD orALD. Subsequently, portions of the conformal dielectric layer locatedoutside of the first undercut regions 64 are removed utilizing aselective etching process. In one embodiment, an isotropic etch such as,for example, a wet etch is performed. After etching, portions of theconformal dielectric layer remained in the first undercut regions 64constitute the first dielectric caps 66.

Referring to FIGS. 14A-14D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 13A-13D after forming asecond opening 72 extending through the insulator portion 58P and theMIM capacitor (52P, 54P, 56P), and in some embodiments, the contactlevel dielectric layer 60, to expose a surface of the ILD layer 30. Thesecond opening 72 also exposes sidewalls of the MIM capacitor (52P, 54P,56P) at a first end of the MIM capacitor (52P, 54P, 56P) (e.g., an endof the second leg of the MIM capacitor). The second opening 72 can beformed by performing processing steps described in FIGS. 11A-11D forformation of the first opening 62.

Referring to FIGS. 15A-15D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 14A-14D after slightlyrecessing the first metal electrodes 52P to provide second undercutregions (not shown) between adjacent capacitor dielectrics 54P andforming second dielectric caps 76 to fill the second undercut regions.The second undercut regions can be formed by lateral etching physicallyexposed sidewalls of the first metal electrode 52P using the processingsteps described in FIGS. 12A-12D for formation of first undercut regions64. Sidewalls of the first metal electrodes 52P are thus offset from thesidewalls of the capacitor dielectrics 54P. The second dielectric caps76 can be formed by performing the processing steps described in FIGS.13A-13D for formation of first dielectric caps 66.

Referring to FIGS. 16A-16D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 15A-15D after forming a firstcapacitor contact structure 82 within the first opening 62 and a secondcapacitor contact structure 84 within the second opening 72. The firstcapacitor contact structure 82 laterally contacts the first metalelectrodes 52P, thereby electrically connecting the MIM capacitor (52P,54P, 56P) to the functional gate 42 of the first functional gate stack40A. The second capacitor contact structure 84 laterally contacts thesecond metal electrodes 56P, thereby electrically connecting the MIMcapacitor (52P, 54P, 56P) to the source/drain interconnect structure 48.

The capacitor contact structures 82, 84 may be formed by depositing aconductive metal within the first opening 62 and the second opening 72.Exemplary conductive metals that can used to provide the capacitorcontact structures 82, 84 include, but are not limited to W, Cu, Al, Co,Ru, Mo, Os, Jr, Rh, and alloys thereof. Any conductive metal depositedon the top surface of contact level dielectric layer 60 may be removedby a planarization process such as, for example, CMP. In someembodiments, a contact liner (not shown) may be formed along surfaces ofeach opening 62, 72 to surround each of the first capacitor contactstructure 82 and the second capacitor contact structure 84. In oneembodiment, each contact liner may include TiN.

Referring to FIGS. 17A-17D, there are illustrated various views of theexemplary semiconductor structure of FIGS. 16A-16D after forming variouscontact structures to FET components. The contact structures includefirst contact structures 90 extending through the contact leveldielectric layer 60 to form contact with the first source/drain contactstructures 46A, a second contact structure 92 extending through thecontact level dielectric layer 60 to form contact with the secondsource/drain contact structure 46B, a third contact structure 94extending through the contact level dielectric layer 60 to form contactwith the third source/drain contact structure 46C, a first gate contactstructure 96 extending through the contact level dielectric layer 60 andthe gate cap 44 of the second functional gate stack 40B to form contactwith the functional gate 42 of the second functional gate stack 40B, anda second gate contact structure 98 extending through the contact leveldielectric layer 70 and the gate cap 44 of the third functional gatestack 40C to form contact with the functional gate 42 of the thirdfunctional gate stack 40C.

When wired to form a crossbar configuration, one of the first contactstructures 90 is coupled to one voltage line, the other of the firstcontact structures 90 is coupled to one current line, the second contactstructure 92 and the second capacitor contact structure 84 are coupledto ground (GND), the third contact structure 96 is coupled to V_(dd),the first gate contact structure 96 is coupled to a first input line,and the second gate contact structure 96 is coupled to a second inputline.

The various contact structures 90-98 can be formed by performingprocessing steps described above in FIGS. 7A-7D for the formation ofsource/drain contact structures 46A, 46B, 46C and the source/draininterconnect structure 48.

While the methods and structures disclosed herein have been particularlyshown and described with respect to preferred embodiments thereof, itwill be understood by those skilled in the art that the foregoing andother changes in forms and details may be made without departing fromthe spirit and scope of the present application. It is thereforeintended that the methods and structures disclosed herein not be limitedto the exact forms and details described and illustrated, but fallwithin the scope of the appended claim.

What is claimed is:
 1. A method of forming a memory cell comprising:providing a first field effect transistor (FET) comprising a firstfunctional gate stack that contacts a portion of a first semiconductormaterial portion and first source/drain regions located on oppositesides of the first functional gate stack in a first active region of asubstrate, a second FET comprising a second functional gate stack thatcontacts a portion of a second semiconductor material portion and secondsource/drain regions located on opposite sides of the second functionalgate stack in a second active region of the substrate, and a third FETcomprising a third functional gate stack that contacts a portion of athird semiconductor material portion and third source/drain regionslocated on opposite sides of the third functional gate stack in a thirdactive region of the substrate; forming first source/drain contactstructures overlying and contacting the first source/drain regions, asecond source/drain contact structure overlying and contacting one ofthe second source/drain regions, a third source/drain contact structureoverlying and contacting one of the third source/drain regions, and asource/drain interconnect structure connecting the other of the secondsource/drain regions and the other of the third source/drain regions,wherein the first, the second and the third source/drain contactstructures and the source/drain interconnect structure are laterallysurrounded by an interlevel dielectric (ILD) layer located over thesubstrate; forming a metal-insulator-metal (MIM) capacitor overlying andcontacting the source/drain interconnect structure and a functional gateof the first functional gate stack, wherein the MIM capacitor islaterally surrounded by a contact level dielectric layer located overthe ILD layer.
 2. The method of claim 1, wherein the MIM capacitorcomprises alternating first metal electrodes and second metalelectrodes, and capacitor dielectrics located between the first metalelectrodes and the second metal electrodes and insulating the firstmetal electrodes and the second metal electrodes from each other.
 3. Themethod of claim 2, wherein the forming the MIM capacitor comprises:forming a MIM stack on the first, the second and the third FETs, thefirst, the second and the third source/drain contact structures, thesource/drain interconnect structure and the ILD layer, the MIM stackcomprising alternating first metal layers and second metal layers andcapacitor dielectric layers separating the first metal layers and thesecond metal layers from each other; forming an insulator layer on atopmost surface of the MIM stack; and patterning the MIM stack toprovide the MIM capacitor and an insulator portion located on a topmostsurface of the MIM capacitor.
 4. The method of claim 3, furthercomprising forming a contact level dielectric layer on the ILD layer,the contact level dielectric layer laterally surrounds the MIM capacitorand the insulator portion.
 5. The method of claim 4, further comprisingforming a first capacitor contact structure contacting the first metalelectrodes at a first end of the MIM capacitor where the firstfunctional gate stack is located and a second capacitor contactstructure contacting the second metal electrodes at a second end of theMIM capacitor wherein the source/drain interconnect structure islocated.
 6. The method of claim 5, wherein the forming the firstcapacitor contact structure and the second capacitor contact structurecomprising: forming a first opening to expose sidewalls of thealternating first metal electrodes and the second metal electrodes andthe capacitor dielectrics at the first end of the MIM capacitor and atop surface of the functional gate of the first functional gate stack;laterally etching the exposed sidewalls of the second metal electrodesto provide first undercut regions; forming first dielectric caps withinthe first undercut regions; forming a second opening to expose sidewallsof the alternating first metal electrodes and the second metalelectrodes and the capacitor dielectrics at the second end of the MIMcapacitor; laterally etching the exposed sidewalls of the first metalelectrodes to provide second undercut regions; forming second dielectriccaps within the second undercut regions; and forming the first capacitorcontact structure within the first opening and the second capacitorcontact structure within the second opening.
 7. The method of claim 3,further comprising forming first contact structures extending throughthe contact level dielectric layer to form contact with the firstsource/drain contact structures, a second contact structure extendingthrough the contact level dielectric layer to form contact with thesecond source/drain contact structure, a third contact structureextending through the contact level dielectric layer to form contactwith the third source/drain contact structure, a first gate contactstructure extending through the contact level dielectric layer and agate cap of the second functional gate stack to form contact with afunctional gate of the second functional gate stack, and a second gatecontact structure extending through the contact level dielectric layerand a gate cap of the third functional gate stack to form contact with afunctional gate of the third functional gate stack.
 8. The method ofclaim 1, wherein the forming the first, the second, and the third FETcomprises: forming a first sacrificial gate stack on the portion of thefirst semiconductor material portion, a second sacrificial gate stack onthe portion of the second semiconductor material portion, and a thirdsacrificial gate stack on the portion of the third semiconductormaterial portion; forming a gate spacer on sidewalls of each of thefirst, the second, and the third sacrificial gate stack; forming thefirst, the second, and the third source/drain regions; and replacing thefirst, the second, and the third sacrificial gate stack with the first,the second, and the third functional gate stack, respectively.
 9. Themethod of claim 8, wherein the forming the first, the second, and thethird source/drain regions comprises: epitaxially growing a firstsemiconductor material on portion of the first semiconductor materialportion located on the opposite sides of the first sacrificial gatestack and on portions of the second semiconductor material portionlocated on the opposite sides of the second sacrificial gate stack whilecovering the third active region; and epitaxially growing a secondsemiconductor material on portions of the third semiconductor materialportion located on the opposite sides of the third sacrificial gatestack while masking the first active region and second active region.10. The method of claim 1, wherein the first, second, and thirdsemiconductor material portions are semiconductor fins.
 11. The methodof claim 1, wherein the first, second, and third semiconductor materialportions are semiconductor nanowires.
 12. The method of claim 1, whereinthe first, second, and third semiconductor material portions are planarportions of a semiconductor material.
 13. The method of claim 1, whereinthe providing of the first FET, the second FET and the third FETcomprises a replacement gate process in which first, second and thirdsacrificial gate stacks are formed, and after forming the first, secondand third source/drain regions, the first, second and third replacementgate stacks are replaced with the first, second and third functionalgate stacks, respectively.
 14. The method of claim 1, wherein the first,second, and third source/drain regions are formed by a selectiveepitaxial growth process.
 15. The method of claim 1, wherein the firstand third FETs are n-type FETs, and the second FET is a p-type FET. 16.The method of claim 1, wherein the first and third FETs are p-type FETs,and the second FET is an n-type FET.
 17. The method of claim 2, whereinthe first metal electrodes comprise two first metal layers, the secondmetal electrodes comprises two second metal layers, and the capacitordielectrics comprises three capacitor dielectric layers.
 18. The methodof claim 1, wherein the MIM capacitor is L-shaped having a first legoverlying and contacting the functional gate of the first functionalgate stack and a second leg overlying and contacting the source/draininterconnect structure.
 19. A method of forming a memory cellcomprising: providing a first field effect transistor (FET) comprising afirst functional gate stack that contacts a portion of a firstsemiconductor material portion and first source/drain regions located onopposite sides of the first functional gate stack in a first activeregion of a substrate, a second FET comprising a second functional gatestack that contacts a portion of a second semiconductor material portionand second source/drain regions located on opposite sides of the secondfunctional gate stack in a second active region of the substrate, and athird FET comprising a third functional gate stack that contacts aportion of a third semiconductor material portion and third source/drainregions located on opposite sides of the third functional gate stack ina third active region of the substrate; forming first source/draincontact structures overlying and contacting the first source/drainregions, a second source/drain contact structure overlying andcontacting one of the second source/drain regions, a third source/draincontact structure overlying and contacting one of the third source/drainregions, and a source/drain interconnect structure connecting the otherof the second source/drain regions and the other of the thirdsource/drain regions; and forming a metal-insulator-metal (MIM)capacitor overlying and contacting the source/drain interconnectstructure and a functional gate of the first functional gate stack. 20.The method of claim 19, wherein the MIM capacitor is L-shaped having afirst leg overlying and contacting the functional gate of the firstfunctional gate stack and a second leg overlying and contacting thesource/drain interconnect structure.